1. Field of the Invention
The present invention relates to flexible flat cables (printed circuits that are flexible) such as FPCs (Flexible Printed Circuits), FFCs (Flexible Flat Cables), and the like. In this specification, a flexible flat cable is generically referred to as an FPC. The present invention relates, in particular, to a connecting structure for a terminal of a double-sided FPC connected to an FPC connector.
2. Related Art
In electronic devices in recent years, flexible flat cables (FPCs) are used for connecting printed circuit boards and electronic component modules mounted in portable information devices typified by, for example, DVCs (Digital Video Cameras), DSCs (Digital Still Cameras), cell-phones, and PDAs (Personal Digital Assistants).
An FPC connector surface-mounted on a printed circuit board (what is called a surface-mounted FPC connector) is provided with: an insulating housing in which an insertion area, into which an FPC is inserted, is formed, and a plurality of contacts mounted side by side at a prescribed pitch in the housing. In order to make the FPC and the contacts touch, for example, a cover-housing that opens and closes is provided at the insertion area.
In increasing the mounting density of FPC connectors that are surface-mounted on a printed circuit board, lowering of mounting height (lowering of profile) and reduction of mounting area are required, and progress is being made with multi-pin and narrow pitch contacts arrayed on these surface-mounted FPC connectors.
As an FPC that is compatible with these types of surface-mounted FPC connectors, double-sided FPCs that realize electrical connections between printed circuit boards by a simplified structure, in comparison to conventional cases, are being invented (for example, see Patent Document 1). The abovementioned flexible circuit boards (double-sided FPCs) are used as cables for interconnecting printed circuit boards, independent devices, and the like.
The double-sided FPC according to Patent Document 1 includes pads for external connections, arranged at two ends of the top face of a base made of polyimide, pads for external connections, arranged at two ends of the bottom face of the base made of polyimide, conductive patterns, electrically connecting, respectively, each pad at the two ends of the top and bottom faces, a gold plating layer applied to the front face of each pad, and a coating layer made of polyimide for protecting the conductive patterns.
Patent Document 1: Japanese Patent Application, Laid Open No. 2002-94203.
FIG. 8 of the present Application is a plan view of the double-sided FPC, showing a first embodiment of Patent Document 1. FIG. 9 of the present Application is a front view of the double-sided FPC showing the first embodiment of Patent Document 1. FIGS. 8 and 9 of the present application correspond to FIGS. 1 and 2 of Patent Document 1.
In FIGS. 8 and 9 of the present Application, the double-sided FPC 50 includes the pads for external connections, arranged at the two ends of the top face of the base 51 made of polyimide, the pads for external connections, arranged at the two ends of the bottom face of the base 51 made of polyimide, the conductive patterns, electrically connecting, respectively, each pad at the two ends of the top and bottom faces, the gold plating layer applied to the front face of each pad, and the coating layer 52 made of polyimide for protecting the conductive patterns.
In FIGS. 8 and 9, at one end of the front face of the base 51, the pads 53a and 53b are arranged, and the gold plating layers 56a and 56b are formed on the front faces of the pads 53a and 53b. At the other end of the front face of the base 51, the pads (not shown in the figure) are arranged, and the gold plating layers 54a and 54b are formed on the front faces of the pads.
The gold plating layers 54a and 54b are connected to a connector arranged on a first printed circuit board, and the gold plating layers 56a and 56b are connected to a connector arranged on a second printed circuit board. The gold plating layer 54a and the gold plating layer 56a are electrically connected, via the conductive pattern 57a. In the same way, the gold plating layer 54b and the gold plating layer 56b are electrically connected, via the conductive pattern 57b. 
In FIG. 9, at one end of the rear face of the base 51, the pads 53c and 53d are arranged, and gold plating layers 56c and 56d are formed on the front faces of the pads 53c and 53d. At the other end of the rear face of the base 51, the pads (not shown in the figure) are arranged, and twin gold plating layers (not shown) are formed on the front faces of the pads.
The gold plating layers 56c and 56d are connected to a connector arranged on the second printed circuit board. The twin gold plating layers formed at the other end of the rear face of the base 51 are connected to a connector arranged on the first printed circuit board. The gold plating layer 56c and one of the gold plating layers formed at the other end of the rear face of the base 51 are electrically connected via a conductive pattern formed on the rear face of the base 51. In the same way, the gold plating layer 56d and the other of the gold plating layers formed at the other end of the rear face of the base 51 are electrically connected via a conductive pattern formed on the rear face of the base 51.
Although only one pair each of the pads and the conductive patterns on the front and rear faces of the base 51 is shown in FIGS. 8 and 9, many pads and conductive patterns may additionally be formed on the front and rear faces of the base 51; and by the pads and conductive patterns formed on the front face of the base 51 being staggered by a half pitch, with regard to the pads and conductive patterns formed on the rear face of the base 51, a narrow-pitch FPC is realized. In these types of narrow-pitch FPCs, pad width is often approximately the same as the conductive pattern width.
FIG. 10 is a front view of a double-sided FPC according to conventional technology in which the width of the pads is approximately the same as the width of the conductive patterns. In FIG. 10, at one end of the front face of the base 61 made of polyimide, a plurality of pads 62a to 62e for external connections is provided, and at one end of the rear face of the base 61, a plurality of pads 63a to 63e for external connections is provided. On these pads 62a to 62e and pads 63a to 63e, the gold plating layer (not shown in the figure), for example, may be formed.
In FIG. 10, the plurality of pads 62a to 62e is disposed at equal intervals (pitch), and the plurality of pads 63a to 63e is also disposed at equal intervals (pitch). By staggering the pitch of the pads 62a to 62e with regard to the pitch of the pads 63a to 63e, by a half pitch in the direction of disposition, a narrow-pitch double-sided FPC is realized.
The double-sided FPC 60 shown in FIG. 10 is connected, for example, to a connector composed of a housing 64 and a cover housing 65. The double-sided FPC 60, which is inserted into a concave region 64a provided in the housing 64, is pressed upon by the cover housing 65 that closes the concave region 64a. At one end that is connected to the connector, referred to as an edge connector, by first contacts provided on the connector, a force P1 acts on each pad 62 to 62e. Moreover, by second contacts provided on the connector, a force P2 acts on each pad 63a to 63e. 
In the double-sided FPC 60 shown in FIG. 10, if continuously pressed upon by the first and the second contacts for a long period of time, it has been ascertained that a deformation remains at both ends of the line in which the pads are disposed on the base 61 made from polyimide. As shown by the dashed lines in FIG. 10, it has been ascertained that one end of the base 61 deforms in the direction in which the force P1 acts, and the other end of the base 61 deforms in the direction in which the force P2 acts.
In the central area of the base 61 shown in FIG. 10, by the pads on the front face and the pads on the rear face reciprocally overlapping through the base 61, the directionally opposed forces P1 and P2 cancel one another out, and deformation of the base 61 is prevented. However, at the ends of the base 61, as in a cantilever beam, if the force P1 or P2 continues to act, and with the passage of time, the elastic deformation limit is exceeded, deformation becomes permanent, and the phenomenon of creep occurs.
In the double-sided FPC 60 shown in FIG. 10, the board thickness of the base 61 is, for example, 25 micrometers, and even when bonding layers, copper foil layers forming the pads, and plating layers (for example, gold plating on nickel plating) are added, the board thickness of the double-sided FPC 60, that is an edge connector, is, for example, 0.12 mm. In order to realize a connector with a low profile of about 1 mm, this type of thin double-sided FPC is required. In the FPC 50 shown in FIGS. 8 and 9, if the base 51 is made thin, similar problems become apparent.
In the double-sided FPC 60 shown in FIG. 10, since inter-electrode walls face against the direction in which the forces P1 and P2 act, deformation of both ends of the base 61 is very slight (for example, of a level that does not affect contact conductivity performance, such as contact resistance with contacts).
However, impression marks due to the contact points of the contacts on the pads 62a and 63e, are different from impression marks due to the contact points of the contacts on the pads 62b to 62e and the pads 63a to 63d. The impression marks on the pads 62a and 63e are shallow, but the impression marks on the other pads are deep. The fact that contact pressure of the contacts differs with positions of the pads is a problem from the viewpoint of reliability.
In the double-sided FPC, in which copper foil is laminated on both faces of an insulating base, the copper foil is etched to form a plurality of conductive patterns, and the conductive patterns are exposed on a terminal for connecting with contacts, a new connecting structure is required that has a structure in which, even if the terminal for the contacts is thin, and without changing board thickness, the two ends of the base do not deform. This topic may be considered an object of the present invention.